1. Field of the Invention
The present invention relates to a capacitive measurement device for producing touch-sensitive and contactless human-machine interfaces, which allows an optimal integration of a large number of measurement electrodes on a detection surface. The field of the invention is more particularly but is non-limitatively that of touch-sensitive and contactless human-machine interfaces.
2. Description of the Related Art
Many devices used for communication and for work use a touch-sensitive or contactless command interface such as a pad or a screen. This type of interface can be found for example in mobile telephones, smartphones, computers with touch-sensitive screens, pads, PCs, mouse devices, touch pads and widescreens etc.
These interfaces frequently use capacitive technologies. The touch surface is equipped with conductive electrodes connected to electronic means which make it possible to measure the variation of the capacitances appearing between the electrodes and the object to be detected in order to carry out a command.
It is possible to provide transparent electrodes which make it possible to superimpose an interface onto a display screen, for example of a smartphone.
Most of these interfaces are touch-sensitive, that is they can detect contact between one or more command object(s) (usually the fingers) and the surface of the interface. Gestural or contactless interfaces are increasingly being developed which are capable of detecting command objects which are further from the interface, without contact with the surface.
The capacitive techniques currently implemented in touch-sensitive interfaces most frequently use two layers of conductive electrodes in the form of rows and columns. The electronics measure the coupling capacitances existing between these rows and columns. When a finger is very close to the active surface, the coupling capacitances close to the finger are altered and the electronics can thus locate the 2D position (XY) in the plane of the active surface.
These technologies, often called “mutual capacitance”, make it possible to detect the presence and the position of the finger through a thin dielectric. They have the advantage of enabling a very good resolution in the location in the plane (XY) of the touch surface of one or more fingers. With appropriate processing software, it is also possible to manage a large number of fingers if the surface of the interface is large enough.
However, these techniques have the drawback of generating on principle large leakage capacitances at the level of the measurement electrodes and of the electronics. In fact the detection of a finger is achieved by measuring the variation in the coupling capacitance created between each row and column of which one is the emitter of an electrical signal and the other is a receiver of the signal to be detected. This signal is proportional to the capacitance between the row and column selected. When a finger is very close to the intersection of the row and column in question, the coupling capacitance is reduced and the finger is detected.
These coupling capacitances, which can be significant even in the absence of the object to be detected, can also drift over time due to ageing, deformation of the materials, or the effect of the variation of the surrounding temperature. These variations can degrade the sensitivity of the electrodes, or can even trigger commands in an untimely manner. This is one of the reasons why these technologies can only detect contact with the finger and not its approach because it is necessary to create a large variation in the capacitance to enable the electric circuit to detect the variation which must be much larger than the drifts to avoid any [capacitance] artefact.
Techniques are also known which make it possible to measure the absolute capacitance which appears between the electrodes and an object to be detected. These techniques are often called “self capacitance”. Document FR 2 756 048 by Rozière is known for example, which discloses a method of capacitive measurement which makes it possible to measure the capacitance and the distance between a plurality of independent electrodes and a nearby object.
This technology uses a guard in order to eliminate all stray capacitance. All the electrodes have the same potential and there is therefore no coupling capacitance between the electrodes that is capable of degrading the capacitance measurement.
This technology is well suited to touch-sensitive and contactless interfaces such as capacitive pads and transparent touchpads and/or small 3D screens, such as the pads of portable computers or the small screens of smartphones. On the other hand, when the pad or the screen is of greater size, the number of electrodes necessary to cover the entire touch surface is too large to be managed by a miniature electronic circuit. And above all, the surface at the periphery of the touch surface and between the electrodes which is necessary for the passage of the tracks connecting the electrodes to the electronics also becomes very significant.
With this technique it is possible to use electrodes in the form of rows and columns in order to minimize interconnection problems. This row-column structure, in which rows and columns are used as independent electrodes, makes contactless or gestural measurement over long distances possible (detection of a finger several centimeters away) but another problem appears when more than one object is to be detected. In fact, it is necessary to scan each row and each column which produces a measurement for virtual objects called ghosts. These ghosts prevent a number of objects from being located absolutely on the touch surface.
The purpose of the present invention is to propose a capacitive measurement device for touch-sensitive and/or contactless interfaces which makes it possible to implement a high number of capacitive measurement electrodes on a detection surface while limiting interconnection problems of these electrodes with the measurement electronics.